1. Field of the Invention
The present invention relates to the measurement of pulsed radio frequency (RF) devices and, more particularly, to the conversion of pulsed RF signals into fixed intermediate frequency (IF) signals and, even more particularly, to a dual channel downconverter for automatically converting the input and output RF signals of a pulsed two-port RF device into fixed IF signals while preserving the relative amplitude and phase of the RF signals.
2. Description of the Background
The task of measuring pulsed two-port RF device parameters with sub-microsecond pulse widths is growing more complex. Simultaneously, the need for test, or measurement, systems combining improved accuracy with reduced cost is also increasing. The present inventors are not the first to address means for measuring pulsed RF signals such as dual channel downconverters. For example, dual channel, or two-band, downconverters are found in U.S. Pat. No. 5,754,951 to Kashima et al., U.S. Pat. No. 5,659,894 and U.S. Pat. No. 5,493,718 to Bayruns et al. and U.S. Pat. No. 5,027,430 to Yamauchi. Additional means for measuring pulsed RF signals are found in U.S. Pat. No. 5,805,460 to Greene et al., U.S. Pat. No. 5,524,281 to Bradley et al., U.S. Pat. No. 5,508,605 to Lo et al., and U.S. Pat. No. 4,611,165 to Nussbaum.
U.S. Pat. No. 5,754,951 to Kashima et al. discloses a microwave mixing circuit of a small size for selecting a signal to be received out of plural microwave signal inputs and a downconverter for converting it to an intermediate frequency signal. A first microwave input signal is applied to a first microwave signal input unit and is guided through a bias terminal from a comparator into a mixer diode (to which a bias current is supplied). The first input signal is converted in frequency by the mixer diode. A second microwave input signal is applied to a second microwave signal input unit and is guided through the bias terminal from the comparator into a mixer diode (to which bias current is not supplied). Hence, the second signal is not converted in frequency by the mixer diode. As a result, only an intermediate frequency signal corresponding to the first microwave signal appears at the intermediate frequency signal output terminal. Unfortunately, this device is intended for simultaneous processing of two oppositely polarized satellite broadcasting/communication signals and is, therefore, wholly inadequate for converting the input and output RF signals of a pulsed two-port RF device into fixed IF signals while preserving the relative amplitude and phase of the RF signals.
U.S. Pat. Nos. 5,659,894 and 5,493,718 to Bayruns et al. disclose an electrical circuit forming a dual-channel low current Low Noise Block (LNB) downconverter comprising two downconverting circuits electrically connected in series with each other and each electrically connected in parallel with a Zener diode such that power consumption is minimized and component life improved. Unfortunately, as with the Kashima et al. device above, these dual channel downconverters were intended for use in the simultaneous processing of two oppositely polarized television signals and cannot be used to convert the input and output RF signals of a pulsed two-port RF device into fixed IF signals.
U.S. Pat. No. 5,027,430 to Yamauchi et al. discloses a low noise converter for satellite broadcast reception incorporating a two-band LNB downconverter for receiving two frequency bands. The low noise converter is capable of receiving a plurality of bands in one converter. Unfortunately, this two-band device was conceived merely to increase the bandwidth capability of downconverters.
U.S. Pat. No. 5,805,460 to Greene et al. discloses a method for measuring the rise/fall time and pulse width of RF pulses using multi-purpose, commercial-off-the-shelf test devices, such as an RF signal down converter, a digitizer and a signal processor. The method is based on digitizing the RF signals and developing an average sample pulse waveform. The rise/fall time and pulse width are then calculated from data points on the sample waveform. Unfortunately, this mathematical method/algorithm requires a local oscillator and fails to provide amplitude and phase measurements.
U.S. Pat. No. 5,524,281 to Bradley et al. discloses a measurement system that comprises a source circuit for receiving feedback signals and providing respective signals at respective discrete frequencies in a prescribed microwave frequency range (wherein the respective provided signals at respective discrete frequencies are substantially phase locked to at least one downconverted signal in response to the feedback signals), a downconverting circuit for linearly downconverting the respective signals and providing the at least one respective downconverted signal, and a phase detector circuit for receiving the at least one respective downconverted signal and for providing the feedback signals. Unfortunately, this device is merely a vector network analyzer that includes a local oscillator.
U.S. Pat. No. 5,508,605 to Lo et al. discloses a method for measuring the frequency of a stream of RF pulses using multi-purpose, commercial-off-the-shelf test devices, such as an RF signal down converter, a digitizer, and a signal processor. The method is based on digital signal processing and determining the zero-crossings of the signal using signal interpolation of the pulse points. Unfortunately, once again, this mathematical method/algorithm requires a local oscillator and fails to provide amplitude and phase measurements.
U.S. Pat. No. 4,611,165 to Nussbaum discloses a method and apparatus for determining the RF carrier frequency of a stream of pulse RF signals. A stream of pulse RF signals is dithered and subsequently fed into a frequency spectrum analyzer. A spectrum analysis of the dithered stream of RF pulses or signals derived therefrom provides improved resolution in the measurement of the RF frequency of the pulse RF signals. Unfortunately, this measurement apparatus provides only frequency data.
Current techniques for measuring the complex response (i.e. amplitude and phase) of a pulsed two-port RF device (e.g. an amplifier) involve complicated and expensive test instruments. A pulsed vector network analyzer system (VNA) is commonly used to measure the complex response of a pulsed two-port RF device by providing a stimulus to the device under test (DUT) and then measuring the amplitude and phase.
However, a pulsed VNA has inherent limitations when used to measure the complex response of a pulsed two-port device that is a subcomponent of a radar transmitter.
FIG. 1 is a schematic representation of a system 10 incorporating a pulsed VNA 12 used to measure the complex response of a pulsed two-port RF device 14 in a radar transmitter. An expensive tunable local oscillator (LO) 16 must be controlled by the VNA controller 18 to downconvert samples of the DUT input signal 20 and output signal 22 to fixed IF signals 24, 26, respectively. The VNA's LO frequency must be constantly controlled to maintain the IF frequency separation between the LO 16 and the RF stimulus 28 to the DUT 14. This limitation usually requires the VNA controller 18 to control the RF stimulus 28 to the DUT 14 and to tune the LO 16 to maintain fixed IF signals 24, 26.
Thus, while the concepts of dual channel downconverters and pulsed RF signal measurement devices are fairly well-known, none of the foregoing apparatus/methods comprise a dual channel downconverter without an associated local oscillator that is specifically intended for measuring and analyzing the complex response (i.e. amplitude and phase data) of pulsed RF signals. It becomes difficult to use a VNA when analyzing a pulsed two-port RF device in a radar transmitter where the stimulus to the DUT is a complex pulsed waveform with frequency tuning and modulation of the RF carrier. This RF stimulus to the DUT is usually already present and it would be undesirable to replace it with the VNA's stimulus. It is equally impractical for the VNA to maintain a fixed IF in the VNA's downconverter because complex frequency tuning and modulation of the VNA's LO is required.
Consequently, it would be greatly advantageous to provide a dual channel downconverter that (1) measures/analyzes the complex response of pulsed RF signals originating in a pulsed two-port RF device in a radar transmitter, (2) does not require a tunable local oscillator, (3) possesses a simple design incorporating durable, commercially available components/devices, and (4) may be economically produced to provide for widespread use.